1. Field of the invention
The present invention relates to variable reluctance machines and, more particularly, to a high specific output variable reluctance drive system incorporating a motor having one or two full-pitch windings for improving current commutation, and a special purpose solid state power converter.
2. Description of the Background
In variable reluctance motors, torque is generated by a rotor and stator which tend to align themselves in order to reach a position of minimum magnetic reluctance. Such motors are simple, efficient, easily controlled, inexpensive, and safe. As a result, there have been significant efforts to develop commercial applications. One evolving application is the stepping motor, which delivers an incremental rotation as the result of a pulsed input. Stepping motors are now widely used in microprocessor and digital control systems, and other systems requiring a low output incremental drive.
Variable reluctance motors have also found limited use in higher power applications such as fan drives, pumps, traction systems, motion control systems, appliances, and general purpose variable speed drives. However, comparatively few industrial applications presently exist for higher power variable reluctance motors. Such industrial needs are satisfied by the more conventional DC and AC induction machines. This is because variable reluctance motors have a special problem: current commutation.
In a continuous drive variable reluctance motor, the current in each phase should be decreased to zero rapidly when the rotor becomes aligned with that phase. This prevents the development of negative torque. It is difficult to accomplish this instantaneous current reduction because the phase inductance is a maximum when the rotor is aligned with that phase. The problem is most serious when the speed of the motor is high.
The prior art uses one of the following two brute force approaches to reduce the current at alignment:
(1) a negative voltage is applied to the phase winding in order to extract the energy trapped in the magnetic field and transfer it to an external storage device such as a battery or capacitor; or
(2) the phase is switched off before the rotor reaches alignment and the energy trapped in the magnetic field is dissipated in a resistor connected to the phase winding through a diode.
Unfortunately, the first technique increases the VA rating and cost of the switching device (a.k.a. converter). Also, the large back-and-forth energy flow between the motor and source causes extra motor and converter losses and creates a need for a large DC bus capacitor.
The second technique reduces the output torque and efficiency of the motor.
Such drawbacks account for the lack of commercial acceptance of switched variable reluctance motors.
It would be greatly advantageous to accomplish nearly instantaneous current reduction in a phase when the rotor is aligned with that phase without likewise increasing the cost of the converter or reducing the output torque of the motor.